As optical communications become popular in recent years, a reduction in cost of an optical communication device is requested. One of the solutions thereof is a method for forming an optical circuit constituting an optical communication device on a large-diameter wafer, such as a silicon wafer, by using a micro optical circuit technique such as silicon photonics. Thus, the material cost per chip can be dramatically reduced to achieve a reduction in cost of the optical communication device.
The examples of typical photodetectors formed on a silicon substrate using such technique include a monolithically-integratable germanium photodetector. FIG. 1 schematically illustrates the structure of a conventional waveguide-combined germanium photodetector. FIG. 2 is the cross sectional view along II-II in FIG. 1. Note that, for ease of understanding of the structure, in FIG. 1 a clad layer 103 and electrodes 116 to 118 illustrated in FIG. 2 are omitted, and only the positions at which the electrodes 116 to 118 are in contact with p++ silicon electrode sections 112, 113 and an n-type germanium region 115, respectively, are indicated by two-dot chain lines.
A germanium photodetector 100 is formed, on an SOI (Silicon On Insulator) substrate composed of a silicon substrate, a silicone oxide film, and a surface silicon layer, using a lithography technique and/or the like. The germanium photodetector 100 includes a silicon substrate 101, a lower clad layer 102 composed of a silicone oxide film on the silicon substrate, a core layer 110 for guiding signal light, a germanium layer 114 that is formed on the core layer 110 and absorbs light, and the upper clad layer 103 formed on the core layer 110 and germanium layer 114.
In the core layer 110, a p-type silicon slab 111 doped with a p-type impurity ion and the p++ silicon electrode sections 112, 113 that are highly-doped with a p-type impurity and act as an electrode are formed. The germanium layer 114 is stacked by epitaxial growth or the like, and has formed thereon the n-type germanium region 115 doped with an n-type impurity. Then, the electrodes 116 to 118 are provided on the p++ silicon electrode sections 112, 113 and n-type germanium region 115, respectively, so as to contact the latter.
Once light enters the core layer 110 and the light is absorbed by the germanium layer 114, then a photocurrent flows between the electrode 117 and the electrodes 116, 118, and therefore the germanium photodetector detects the light by detecting this current.